I'm looking to invest in BTSX for the long term. Is there any way to do some sort of cold storage?
Are you aware that in the near future with your BSTX you can buy BitAssets which pay interest? You'd get compound interest by investing properly through Bitshares X which is far better than cold storage. You could even diversify and hedge your bets by holding Bitcoin within the Bitshares X wallet.
Bytemaster can give you the details on compound interest, Bitassets, and their benefits. Cold storage may not be necessary but there does need to be multisig wallets or something.